Apparatus for pressure casting by direct fluid pressure

ABSTRACT

For casting a linear strip fill in a longitudinal slot, such as in a stack of aligned peripherally slotted laminations, to form cast metal bars, the stack is disposed in a chamber space of a close fitting main enclosure whose confining wall inner surface is congruent with the virtual desired external contour of the bars to be cast in the slot, so no further shaping or machining of the bar is necessary. The main enclosure is open at its front end as a vent to establish a high pressure gradient for the molten metal set into the other end at high pressure. The molten metal is held in an auxiliary chamber by a frangible disc and high pressure on the molten metal fractures the disc and forces the molten metal into the main chamber at high pressure that makes the location of the vent important in this case, and useful in aiding the movement of the molten metal into the slots to be filled. One guide in front of the body to be filled helps to guide the high-pressure metal. A second guide behind the body directs excess metal away from the body in individual pin type bars for subsequent easy removal.

. United States Patent [72] Inventor Vincent J. Forras 14-14 Utopia Parkway, Beechhurst,

Queens, N.Y. 11357 [21] Appl No. 739,903 [22] Filed June 14, 1968 [45] Patented Feb. 2, 1971 Continuation of application Ser. No.

373,213, June 8, 1964, now abandoned.

I 54] APPARATUS FOR PRESSURE CASTING BY DIRECT FLUID PRESSURE 1 Claim, 11 Drawing Figs.

[52] US. Cl. 164/304, 164/1 19 [51] 1nt.Cl B22d 17/06 [50] Field of Search 164/98.

[56] References Cited UNIT ED STATES PATENTS 1,561,233 11/1925 Hoey 164/109 3,163,897 1/1965 Sylvester 164/119 3,196,501 7/1965 Balevsk etal....... 164/119 FOREIGN PATENTS 676,708 1 H1929 France 164/133 1.127.545 4/1962 Germany 164/136 779,723 7/1957 Great Britain 164/306 Primary Examiner-J. Spencer Overholser Assistant Examiner--R. Spencer Annear Attorney-Julius E Foster ABSTRACT: For casting a linear strip fill in a longitudinal slot. such as in a stack of aligned peripherally slotted laminations, to form cast metal bars, the stack is disposed in a chamber space of a close fitting main enclosure whose confining wall inner surface is congruent with the virtual desired external contour of the bars to be cast in the slot, so no further shaping or machining of the bar is necessary. The main enclosure is open at its front end as a vent to establish a high pressure gradient for the molten metal set into the other end at high pressure. The molten metal is I held in an auxiliary chamber by a frangible disc and high pressure on the molten metal fractures the disc and forces the molten metal into the main chamber at high pressure that makes the location of the vent important in this case. and useful in aiding the movement of the molten metal into the slots to be filled. One guide in front of the body to be filled helps to guide the high-pressure metal. A second guide behind the body directs excess metal away from the body in individual pin type bars for subsequent easy removal.

PATENTED ra. 2'91! sum 1 OF v2 n we 7 I PRESSURE SUPPLY INVENTOR mvcEA/T .r. FOQDAS ATTORNEX P'ATENTEDE EHQQYI 3559.721

' snmaorz v ,IIIIJIIIIIIIIIII I l 'IIIIIIII'IIII "ll-II lllllllll XI VINCENT J. FURRAS INVENTOR.

ATTORNEY APPARATUS FOR PRESSURE CASTING BY DIRECT FLUID PRESSURE This application is a continuation of my application Ser. No. 373,213 filed June 8, 1964, and abandoned in favor of this application and after the filling of this application.

This invention relates to a method of, and an apparatus for, metal casting, for forming molten metal into predetermined shapes, particularly on preformed structures, or in preshaped molds, and, as a corollary, for one example, to form a squirrel cage structure on a stack of laminations, to constitute a component for a rotor of an induction motor.

In the evolution of instrumentation and servo control systems, induction motors of the squirrel cage type for use in such systems have been made progressively smaller and smaller, to dimensions as small as one-half inch in lamination stack length, thus determining slot length, and three-eights of an inch in lamination diameter, with peripheral slots one-sixteenth of an inch deep and as narrow as twenty-thousandths of an inch.

Those small dimensions have presented a considerable problem in forming a squirrel cage electrically satisfactory in its various characteristics, to assure proper operation of the motor consistent with its design parameters.

The small dimensions of the slots have substantially compelled resort to molten metal for introduction into, and placement in, such slots to form the bars of the desired squirrel cage.

The use of molten metal introduces certain other problems. One problem is that molten metal tends to adsorb and absorb air as a gas and to entrain such air as small bubbles. When such molten metal is fed into the rotor slots in the lamination stack, the existence of a bubble in a slot bar is usually enough to destroy the mechanical form or continuity in the bar and thus render the bar electrically ineffective. The squirrel cage is therefore incomplete and the rotor is not properly operative and is unsatisfactory for normal intended use. To solve that problem, the molten metal is first subjected to a high pressure to express any included air bubbles before the molten metal is fed to the mold compartment for casting disposition.

Another problem that is introduced by the use of molten metal is how to control and hold the molten metal while in a temporary or dwell compartment, and then how to effect quick transfer of the molten metal into the mold compartment for final casting disposition. The problem is one of valving. To provide such valving operation, a disc of frangible construction of material to resist the high temperature of molten metal is used to hold the metal against displacement, until displacing force is applied, suddenly and with high pressure. The disc is thus ruptured and the metal forced into the mold compartment.

When dealing with magnetic metal, it is necessary to avoid heating the metal to a temperature that would affect its magnetic qualities. Avoidance of such temperature presents an additional problem. Therefore, to solve that problem in accordance with this invention, the introduction of the high-temperature molten metal is accomplished at high speed into the mold compartment and into or onto any structural element into or onto which the metal is to be formed and bound. Thus, an ample amount of metal is supplied, quickly, while still fluid, to fill all desired spaces, and the cooling or chilling is also accomplished rapidly by the structural element and the mold. Ample metal is thus present to prevent substantial shrinkage.

By way of example, in a particular application to which the invention may be applied, the apparatus and the method are so constructed and employed as to be particularly useful in forming a squirrel cage structure in place on a stack of laminations on a shaft for use as a rotor for an induction motor. As formed by the process herein, the lamination stack with the squirrel cage cast thereon will require only minor handling for machining and trimming operations.

One object of the invention therefore is to provide an apparatus and a method of casting whereby a squirrel cage structure may be formed directly on a stack of laminations on a shaft, to form a rotor structure requiring a minimum of subsequent handling and machining to place the squirrel cage structure in final condition on the rotor.

Another object of the invention is to provide a casting apparatus and a method of casting wherein a charge of molten metal is transferred from a temporary storage or measuring chamber into a molding chamber at relatively high speed.

Another object of the invention is to provide a novel valve 7 element for use in confining the molten metal charge in a holding chamber until the operation is performed of transferring the molten metal from that holding chamber into the molding chamber.

Another object of the invention is to provide a gate element for use between the holding chamber and the molding chamber which serves as a transfer barrier until the moment of desired transfer of the molten metal, whereupon the barrier may be quickly fractured to provide an open passage for quick transfer of the molten metal from the holding chamber into the molding chamber.

In order to effect the fast operation of transferring the molten metal from the holding chamber into the molding chamber, a fast-acting high pressure force is applied to the charge of molten metal in the holding chamber. Such force is of high-pressure value and of steep front for fast transient approach to its high-pressure steady-state value.

Such impressed force quickly fractures the frangible disc that is normally disposed to cover the opening to the transfer passage between the holding chamber and the molding chamber. At the same time as the disc is fractured at said opening, the molten metal is accelerated to high speed by such high-pressure force, and at such high speed fills the spaces in the mold form designed to receive such molten metal. The metal is under high pressure and freezes quickly, thereby assuring a fully filled space, and freezing while such space is pressed full, thereby avoiding shrinkage and thereby holding to the design dimensions.

The mold or mold form may be held at relatively low temperature, for example, as low as 500 F., with assurance of full and complete casting, as a result of the high-speed action and movement of the fluid metal.

In order that the high-pressure fluid, for convenience referred to as air, shall impress a uniform pressure on the top exposed surface of the charge of molten metal, the air should be introduced into the charge holding chamber with minimum tendency to orient and concentrate on any small area, such as the axial area if the airstream entered axially into such holding chamber. To establish an optimum uniform air-pressure condition on the top surface of the molten charge, the air stream is introduced in several subdivided streams oblique to the axis perpendicular to the top surface of the metal charge.

The construction of an apparatus in accordance with the invention and the method of operating the apparatus in accordance with the invention are explained in the following specification, taken in connection with the accompanying drawings, in which:

FIG. 1 is a vertical sectional view of a simple apparatus embodying and functionally illustrating the invention;

FIG. 2 is a vertical sectional view, partially in section and partially in elevation, illustrating the apparatus of the invention as arranged to include a mold for accommodating a rotor lamination stack on a shaft to receive a squirrel cage structure to be formed thereon by the casting operation of the invention;

FIG. 3 is an end elevational view, partly in perspective, of the mold in FIG. 2;

FIG. 4 is a plan view of the bottom part of the mold of FIG.

FIG. 5 is a plan view of the top part of the mold looking upward into the mold of FIG. 2;

FIG. 6 is a perspective view of a stack of laminations mounted on a shaft and designed to operate as a rotor of an induction motor when the slots of the laminations are appropriately filled with bars connected to end rings of a squirrel cage structure to be formed on the laminations;

v FIG. 7, after all excess metal has been trimmed from the two end rings, and the rotor is otherwise ready for the final grinding or polishing operation;

FIG. 9 is a vertical section taken through the finished structure of the rotor in FIG. 8, to illustrate the manner in which the end rings are formed to join the ends of the respective bars at each end of the rotor;

FIG. 10 is a schematic showing of a metal-holding chamber in which a slug of metal is heated, as by high frequency induction, to a desired temperature and then expressed by highpressure fluid, such as gas or hydraulic liquid; and

FIG. 11 is a schematic functional view of an arrangement showing how a plurality of casts may be made simultaneously from the metal-holding chamber, and into vertical molding chambers.

As shown in FIG. 1, a casting machine 20 is shown as functionally comprising a cylinder 22 having a cylindrical chamber 24 symmetrical about a vertical axis 26 and being open at the top for receiving a charge of metal, as from a ladle 28, and having a cooperating sealing means 30 for connecting an external source 32 of high-pressure fluid, for example, compressed air, to said chamber 24 with a suitable holding arrangement on said sealing means 30 to permit the cylinder chamber 24 to be sealed off from the ambient atmosphere during a certain phase of the operation of the apparatus, when the high-pressure fluid is injected into the chamber 24. The sealing means 30 is pictured as a piston disposed and arranged to be operated by a piston rod 38 connected to a suitable pressure force-operating device 40, and provided with an axial bore 39 to transmit the high-pressure fluid or air. A plurality of auxiliary passages 41 communicate with the axial bore 39 and open out at the bottom surface 30a of the pistonlike sealing means 30.

The cylinder 22 is provided with a bottom wall or floor 42 with a small central opening 44 which may be coaxial with the cylinder 22 to serve as an outlet port from the cylinder 22. The outlet port 44 communicates with a transfer conduit 46 which ends in a port 48 communicating with a space 50 serving as a molding chamber for receiving an object 52, represented schematically, with respect to which the casting operation is to be performed. The molding chamber 50 is confined within a molding enclosure 54 provided with functional vents indicated at 56 and 58, so there will be a minimum of back pressure on any molten metal that is directed into the molding space 50, onto or against the body 52. It will be realized, of course, that instead of using the venting spaces 56 and 58 for direct venting into the outer atmosphere, those two venting passages 56 and 58 may be connected to a suitable vacuum system tem porarily to reduce the air pressure in the molding space 50 to a relatively low pressure value, so that movement of the molten metal during the casting operation would be effectively free from any back pressure from any air in the molding chamber 50.

When a casting operation is to be performed in accordance with the principles of this invention, a charge of molten metal 60 will be poured into the cylinder 22, as from the ladle 28 for example. In order to be able to control the timing of the transfer of the molten metal charge 60 into the molding space 50, and in order to permit an appropriate pressure force to be imposed on the metal for propelling the metal through the conduit 46 into the molding chamber 50, it is appropriate and necessary to hold the metal charge 60 in position until the propelling pressure is to be applied.

It will be recognized that this operation of holding the molten metal in position and then releasing it and propelling it at a desired time represents a valving operation that must be such as to provide a static condition for holding the molten metal 60 in place, and a very highly mobile condition that will permit the molten metal to be propelled at a very highly accelerated rate, that constitutes a relatively high rate impulse action.

The element that is utilized for such valve-controlling action is a very important constituent element of the invention, both as to its character and disposition as a static element, and as to its characteristic structure which permits it to function extremely rapidly in moving out of the way of the molten metal to permit the molten metal charge to be quickly accelerated to a high speed for movement into the molding chamber 50.

As such astatic and dynamic valve element in the operation of this invention, a disc 65 is employed, consisting of a refractory material arranged in frangible condition, consisting, for example, of a layer or thin batt of fiberglass strands, arranged in random fashion but disposed in close enough physical relation to form and constitute a nonwoven layer with spaces or interstices to small to permit a layer of molten metal on one side of the disc to globulize and flow through such layer of fiberglass strands under ordinary gravity or ambient atmosphere pressure.

This disc 65 thus serves to provide sufiicient strength in its static unstressed condition to support a quantity of molten metal 60 representing a single charge in the cylinder 22.

In the performance of a casting operation, a sudden impulse pressure is impressed upon the charge of molten metal 60. Such pressure is transmitted through the mass of molten metal 60 to the valve disc 65 of frangible material, and thereupon a small coaxial central area 65A of the valve disc 65, directly above the inlet port 44 to the transfer conduit 46, is sheared away at one region of its circumference, coextensive with the circumferential corner 66 outlining and defining that inlet port 44. As the small area 65A of the disc is thus sheared and ruptured at one point of the circumferential area, the pressure on the charge of molten metal forces the small volume of mo]- ten metal that is directly adjacent and above the opening or inlet port 44 to move into and through the communicating passage 46 into the casting chamber 50. The operation has been observed to be such that when the valving area 65A is partially torn away from the body of the disc 65 at an arcuate portion of the corner 66, the valving area 65A is not entirely tom away but is pressed against the inner surface of the conduit 46 by the onrushing molten metal, and thus does not carry the material of the valving area 65 along in the stream of molten metal passing into the chamber 50.

Because of the hydraulic principles that are involved, the ratio of the diameter of the cylinder 22 to the small diameter of the transfer conduit 46 causes the moving molten metal to move through the conduit 46 at a high velocity.

The impulse pressure by means of which the action on the mass of molten metal 60 is established consists of energy of two types, potential and kinetic. The source 32 of compressed air at high pressure is connected to the chamber 24 to inject the high-pressure airstreams into the chamber 24 to impress a high static pressure and kinetic pressure on the metal charge 60. The high pressure of the air streams serves to impose a high-speed shock pressure on the charge of molten metal 60. The rupture of the frangible disc 65 results, and the continuing pressure on the metal 60 causes the consequent expulsion of the molten metal 60 from the cylinder 22 into the molding chamber 50.

In FIG. 2 is shown a modified arrangement of apparatus utilizing the apparatus and principles of operation as explained in connection with FIG. 1. As shown in FIG. 2, the invention is employed in an apparatus including a molding chamber in which a molding die set 70 consisting of an upper molding die 72 and a lower molding die 74 are utilized in combination, to define a molding chamber in which a rotor body with a stack of laminations may be suitably disposed to receive a high-speed charge of the molten metal 60 from the cylinder 22 in order to form a squirrel cage structure on the stack.

As shown in more detail in FIGS. 3, 4 and 5, the upper molding die 72 and the lower molding die 74 are respectively provided with longitudinal fluted sections 72A and 74A of semicircular cross section, disposed longitudinally of the two dies 72 and 74. 4

The upper die 72 is also provided with a transfer passage 76 the upper end of which represents an input port 76A in communication with the compartment 24 in cylinder 22 through a valve disc 65 similar to that in FIG. 1, all coaxial with the vertical central axis 26 of the equipment, as illustrated both in FIG. 1 and in FIG. 2.

In the upper molding die 72 the passage 76 communicates with the semicircular flute 72A. In the lower molding die 74 the longitudinal flute 74A is of semicircular section so that the flutes 72A and 74A will define a substantially cylindrical space 75 when the upper and the lower molding dies 72 and 74 are superposed in surface-to-surface relationship and held in such position tightly, by suitable clamps not shown, so that any molten metal forced into the riser conduit 76 of the upper molding die 72 will be confined to the space defined by the fluted surfaces 72A and 74A between the two dies 72 and 74 and will not be permitted to flow out between the engaging surfaces between the two molding dies 72 and 74.

The two molding dies 72 and 74 serve to illustrate one manner in which the casting operation may be utilized to cast a squirrel cage 80 directly on a stack of laminations 82 mounted on a shaft 84, so that the subsequently finished article will constitute a rotor for a small induction motor such as a servomotor.

FIG. 6 shows a stack of laminations 82 supported on a shaft 84 with the rotor slots 86 angularly disposed relative to the axis of the shaft, in a manner and for reasons well known to induction motor designers. For the present purpose it is sufficient merely to call attention to the fact that the slots are so disposed.

In the formation of a squirrel cage 80 on a rotor stack 82 of the form shown in FIG. 6, not only must the slots 86 be filled with metal to serve as conducting bars 88 from end to end through the slots, but all of those bars 88 must be joined at both ends by common end rings 92 and 94, such as shown in FIGS. 8 and 9 as the end result.

In order to provide for the formation of the squirrel cage 80, a stack of rotor laminations 82, such as shown in FIG. 6, is disposed in the bottom molding die 74 in the manner shown in FIG. 4. The diameter of the lamination stack 82 of FIG. 6 is substantially equal to the diameter 75 of the cylindrical space defined by the two flutes 72A and 74A in the two molding dies 72 and 74.

A lamination stack 82, such as shown in FIG. 6, is supported between two cylindrical supports 96 and 98, each of which is provided with a coaxial bore 96A and 98A to receive the two ends 100 and 102 of the shaft 84. The outer diameters of the two cylindrical supports 96 and 98 are also such as to fill the space measured by the diameter 75 when the two molding dies 72 and 74 are joined.

The supporting spacer 96 is provided also with two hubs 104 and 106, both coaxial, and the hub 104 has such outer diameter as to provide an annular space 110 between the hub periphery and the outer periphery of the flute surface and the end face surface of the end lamination of the stack 82 when the cylindrical support is placed tightly against the stack as shown in FIG. 4. This annular space 110 will define the annular region at each end of the bars, within which the end ring 94 of the squirrel cage 80 will be formed. The molten metal that is forced downward through the passage 76 will fill the annular space 110 to form first the end ring 94 at that end of the lamination stack 82.

At the other end of the lamination stack, a similar cylindriplane 120 perpendicular to the axis 116, indicated by the line 120. The annular ring 92 that will be formed in the space 114 between the stack and the support 98 will similarly be symmetrical with respect to a plane indicated by the dotted line 120A perpendicular to the common axis 116.

The supporting cylinder 98 at the left-hand .end of the lamination stack 82 is provided with several axial slots 124 along and in its peripheral surface, extending the full length of that peripheral surface of the cylindrical support 88 in order to provide a plurality of venting spaces into which excess 1110]- ten metal may run after filling the space 114 to form the end ring 92 of the squirrel cage on the lamination stack.

After the end ring 92 is formed by the metal that has flowed through the slots 86 into the annular ring space 114, any excess of metal beyond that can continue its forward movement into and through those longitudinal grooves 124 in the peripheral surface of the cylindrical support 98, where such excess metal will solidify and form long slender tongues 128 of solidified metal, FIG. 7, due to the chilling effect of the sidewall surfaces of those grooves 124.

At the same time, the metal in the lamination slots 86 will be compressed, to fill those slots, and will be chilled to solidification to constitute the bars of the squirrel cage, and the metal in the two end rings 92 and 94 will also be solidified while under pressure. The result of such solidification under pressure is that there is substantially no shrinkage. There is therefore substantial uniformity in products made by this process.

After the metal in the squirrel cage has chilled and solidified, the cylinder 22 is removed from the molding die set 70, and the excess metal 130 in the transfer passage 46 is broken away from the metal mass 60 which by now has also solidified. The excess metal 130 is removed from the end ring 94 and the tongues 128 are removed from the end ring 92, leaving the rotor 82 in condition for final sizing by suitable grinding. When the rotor is thus finally completed, the bars are clearly defined, as are also the end rings 92 and 94, as

shown in FIG. 9.

Any excess metal of the charge '60 in the cylinder 22 is removed for the subsequent operation, and a new frangible disc 65 placed in position in the circular recessed seat 73 in the top of the top molding die 72, and the cylinder 22 reseated in said circular seat 73 with a tight fit. A new charge of molten metal is ladled into the cylinder 22 for a new lamination stack 82 which has means meanwhile been assembled with its supports in the molding die set 70.

In the preferred form of the apparatus and method as shown and described, the operating force has been derived from compressed air. Such force may be obtained from sudden hydraulic fluid action or from individual explosive charges in the cylinder 22 above the metal charge.

Also, instead of pouring molten metal into the cylinder 22, an apparatus as in FIG. may be employed, in which a slug may be placed in a cylinder 142 of dielectric or nonmetallic material, such as quart-z, for example, with an induction coil 144 to heat and melt the metal for the pressure-forcing operation, as used in FIG. 1. The valve 'disc 65 is used as in FIG. 1 and the outlet or transfer passage conduit 46 may be appropriately connected to a molding chamber such as 50 in FIG. 1.

The apparatus of FIG. 1 may be utiliied for a metallizing operation, where the body 52 is to be so treated. In that case the body 52 may be surrounded by a suitable fence 150, appropriately vented at 152 so the projected metal can freely engage and sweep over the exposed surface of the body 52 where the metallizing is desired.

The apparatus and method disclosed herein may be readily applied for multimold operation by the provision of more than one outlet passage from the pressure chamber 24 in which the fluid metal is held until the high-pressure operation is performed.

A vertical arrangement of a molding chamber may provide certain advantages-and flexibilities in operation.

In FIG. 11, such vertical disposition 18 indicated, utilizing the general arrangement of mold structures of FIGS. 2 to 5, with supports for the shaft ends of a stack of laminations, similar to those in FIG. 4. Two such mold structures 160 and 162 are shown connected to the one cylinder 22, to illustrate both the vertical disposition for closer placement, and the multiple arrangement fed from one metal chamber, such as 24 of FIG. 1. One valve disc 65 suffices for the plurality of transfer passages from the fluid metal charge to the several molding die sets.

It will be understood that the invention is not limited to the details illustrated herein, nor to the use of molten metal, since a fluid thermoplastic that sets upon cooling may be treated as taught herein for surfacing or forming, and the structural details may be modified within the spirit and scope of the invention, as defined in the claims.

Iclaim:

1. Casting apparatus comprising:

a main enclosure as a mold defining a main chamber for ac commodating a body on which a casting is to be formed, said enclosure having an inlet port to receive a charge of molten metal to form the casting, and having a vent to serve as an outlet port for air from the enclosure;

an auxiliary enclosure defining a cylindrical space with relative large diameter to temporarily hold an introduced transitory charge of molten metal to be propelled into said main chamber, said auxiliary enclosure having an input opening through which said charge of molten metal may be introduced and an exit port of small diameter relative to said large diameter;

means for sealing said input opening to withstand a high pressure within said auxiliary enclosure;

a connecting conduit of relatively small diameter coaxially disposed between said exit port from said cylindrical space of said auxiliary enclosure and said inlet port to said main chamber of said main enclosure to provide a relatively short path between said space and said chamber;

a sheet of frangible material disposed in the cylindrical space of said auxiliary enclosure to temporarily support such transitory charge of molten metal and to normally prevent such charge from entering said connecting conduit under gravity force alone on said molten metal;

means for impressing a sudden high-amplitude shock pressure force on such transitory charge of supported molten metal sufficient to rupture said sheet of frangible material to provide an opening to the connecting conduit and to force said molten metal to move at accelerated speed through said connecting conduit and into said main mold chamber, said force-impressing means being operative to maintain said pressure force uniform at its high amplitude value during the full period of movement of said metal through said connecting conduit, said shock-pressure force means including a piston axially movable into said cylindrical space to seal said space before impressing said shock pressure force, and also including an external reservoir of air compressed to high pressure; and

valve means for injecting such high-pressure air into said cylindrical space to put high pressure onto the top surface of said charge of molten metal in said space to rupture said frangible sheet and force the molten metal at high speed through said connecting conduit. 

1. Casting apparatus comprising: a main enclosure as a mold defining a main chamber for accommodating a body on which a casting is to be formed, said enclosure having an inlet port to receive a charge of molten metal to form the casting, and having a vent to serve as an outlet port for air from the enclosure; an auxiliary enclosure defining a cylindrical space with relative large diameter to temporarily hold an introduced transitory charge of molten metal to be propelled into said main chamber, said auxiliary enclosure having an input opening through which said charge of molten metal may be introduced and an exit port of small diameter relative to said large diameter; means for sealing said input opening to withstand a high pressure within said auxiliary enclosure; a connecting conduit of relatively small diameter coaxially disposed between said exit port from said cylindrical space of said auxiliary enclosure and said inlet port to said main chamber of said main enclosure to provide a relatively short path between said space and said chamber; a sheet of frangible material disposed in the cylindrical space of said auxiliary enclosure to temporarily support such transitory charge of molten metal and to normally prevent such charge from entering said connecting conduit under gravity force alone on said molten metal; means for impressing a sudden high-amplitude shock pressure force on such transitory charge of supported molten metal sufficient to rupture said sheet of frangible material to provide an opening to the connecting conduit and to force said molten metal to move at accelerated speed through said connecting conduit and into said main mold chamber, said forceimpressing means being operative to maintain said pressure force uniform at its high amplitude value during the full period of movement of said metal through said connecting conduit, said shock-pressure force means including a piston axially movable into said cylindrical space to seal said space before impressing said shock pressure force, and also including an external reservoir of air compressed to high pressure; and valve means for injecting such high-pressure air into said cylindrical space to put high pressure onto the top surface of said charge of molten metal in said space to rupture said frangible sheet and force the molten metal at high speed through said connecting conduit. 